RF spectrum analyzers are known in the art and considerable interest presently exists in such analyzers which are fabricated using integrated optics processes. For a better understanding of these systems and the components thereof, see "Design, Fabrication and Integration of Components for an Integrated Optic Spectrum Analyzer," by M. K. Barnoski et al, 1978 Ultrasonics Symposium Proceedings, IEEE, and "Diffraction-Limited Geodesic Lens for Integrated Optics Circuits," by B. Chen et al, Applied Physics Letters, 33(6), Sept. 15, 1978.
Generally, optical spectrum and analyzers include a laser source, collimating optics, focussing optics, a detector array and a transducer for modulating laser light as a function of the RF frequency of the signal applied to the transducer. Ideally, the laser source provides light generated by the process of stimulated emission which is at a single wavelength and fixed polarization. However, in all semiconductor lasers, such as gallium aluminum arsenide, spontaneous emissions occur which are broadband in wavelength and have mixed polarization.
The spontaneous emission problems result in optical noise being introduced into the system thus reducing the dynamic range of the system. Prior art solutions to this problem have involved attempts to deposit a multi-layer interferometric filter directly on the detector array to filter out the spontaneous emission signals. This, however, is a very difficult task from a manufacturing standpoint. Moreover, the frequency filtering performance of the interferometric filter is severely degraded for a converging beam impinging upon the detector array.
Additionally, where such spectrum analyzers are fabricated on an integrated circuit chip using integrated optics techniques, system performance is limited by the length of the crystals used as a substrate material. For instance, lithium niobate crystals cannot be grown along one crystal lattice axis (X-axis), while growth along the other two axes may be on the order of 10 inches, or so.
The frequency resolution of the analyzer is substantially determined by the focal length of the focussing optics and its detector size. Current state of the art detectors are generally used in these analyzers and have an element size on the order of 8 microns, which is the limit of present detector technology. Thus, to increase the frequency resolution it is necessary to increase the focal length of the focussing optics given the limited chip size available. For lithium niobate crystals, for example, the maximum length attainable along the X-axis is about 23/4 inches. The acoustic waves must propagate along the Z-axis, and thus the collimating and focussing optics are arranged along the X-axis, thus limiting the maximum focal length to about one inch.